The present invention relates generally to a method and apparatus for the separation of at least one gaseous component from a mixture of gaseous components from a feed mixture. More specifically, the present invention relates to a method of separation of a gaseous component from a mixture of gases contained in a gaseous feed stream.
In the past, various techniques have been employed for the separation of components from gaseous mixtures, such as by chemical and physical absorption techniques. These techniques involve the dispersion of a gas in the absorbent liquid as bubbles or gas flowing counter-current to a liquid which is in the form of drops or a thin film in spray towers, packed towers, and the like. Such devices, however, suffer from flooding, loading, entrainment, and the like. The recovery of condensable hydrocarbons from natural gas, on the other hand, is achieved by highly energy intensive cryogenic processes such as refrigerated absorption in heavy oils or cryogenic distillation.
A less energy intensive process which can be an alternative to the ones mentioned above is membrane separation. Gas separations using polymeric membranes have met with some success (T. E. Cooley and W. L. Dethloff, "Field Tests Show Membrane Processing Attractive", Chem. Eng. Prog., Vol. 81, p.45 (1985)). However, the wider use of polymeric membranes for gas separations has been inhibited by the low selectivity and permeability values they exhibit (S. L. Matson et al., "Separation of Gases with Synthetic Membranes", Chem. Eng. Sci., Vol. 38, p.503 (1983); A. Sengupta and K. K. Sirkar, "Membrane Gas Separation", Progress in Filtration and Separations, R. J. Wakeman, Ed., Elsevier, Amsterdam, Vol.4, p.289 (1986)). V. T. Stannett, "The Transport of Gases in Synthetic Membranes--an Historic Perspective", J. Mem. Sci., Vol. 3, p.97 (1978) has provided a historical perspective on gas transport through polymeric membranes. Liquid membranes, on the other hand, are known to possess high species permeabilities and selectivities in gas permeation (S. G. Kimura et al., "Industrial Applications of Facilitated Transport", Recent Developments in Separation Science, N. N. Li, Ed., CRC Press, West Palm Beach, Fla., Vol.5, p.11 (1979); M. S. Brennan et al., "Natural Gas Separation Using Supported Liquid Membranes", AIChE J., Vol. 32, p.1558 (1986) and M. S. Brennan et al., "The Use of Swollen Silicone Rubber Membranes for Natural Gas Separations", ICOM '87, Tokyo, Japan (1987); S. Majumdar et al., "A New Liquid Membrane Technique for Gas Separation", AIChE J., Vol. 34, p.1135 (1988); K. M. Kaka et al., "Natural Gas Separation by Oil-Swollen Membranes", Proc. Int. Mem. Tech. Conf., Sydney, Australia (1988)). Furthermore, using the principle of facilitated transport even higher species selectivities and permeabilities may be achieved (W. L. Ward, "Immobilized Liquid Membranes", Recent Developments in Separation Science, N. N. Li, Ed., CRC Press, Cleveland, Ohio, Vol. 1, p.153 (1972); R. D. Hughes et al., "Olefin Separation by Facilitated Transport Membranes", Recent Developments in Separation Science, N. N. Li, J. M. Calo, Eds., Vol. 9, CRC Press, Boca Raton, Fla. (1986)). An early use of a liquid as a separation barrier for gas separation was achieved by immobilizing it in the pores of a microporous substrate where it was kept in place by capillary action of the micropores, these types of membranes were called immobilized liquid membranes (ILM) (W. L. Ward, "Immobilized Liquid Membranes", Recent Developments in Separation Science, N. N. Li, Ed., CRC Press, Cleveland, Ohio, Vol. 1, p.153 (1972)). The liquids used as membranes are chosen on the basis of gas solubility, selectivity and volatility. The microporous support is chosen on the basis of its inertness, small pore size, thinness, and the like. This ILM technique along with few other variations have several deficiencies for gas separation, the most important ones being poisoning (Matson et al., "Separation of Gases with Synthetic Membranes", Chem. Eng. Sci., Vol. 38, p.503 (1983)), loss of evaporation and the problem of humidity control (S. G. Kimura and G. F. Walmet, "Fuel Gas Purification with Permselective Membranes", Sep. Sci. Tech., Vol. 15, p. 1115 (1980)), resulting in flooding of the aqueous membrane.
More recently, however, Majumdar et al., "A New Liquid Membrane Technique for Gas Separation", AIChE J., Vol. 34, p.1135 (1988) developed a new liquid membrane technique known as the contained liquid membrane (CLM). Here a thin film of water or an aqueous solution is contained in the space between two thoroughly mixed sets of hydrophobic microporous hollow fibers which carry the feed and the permeate streams respectively. Since water or an aqueous solution is acting as the liquid membrane, and since it is contained between two sets of hollow microporous hydrophobic fibers, this technique is designated hollow fiber contained liquid membrane (HFCLM). The aqueous liquid membrane, which is immobilized in the shell side of the permeator, is maintained at a pressure higher than those of both gas streams, feed and permeate, to prevent physical mixing of the two gas streams. Using this technique, Guha et al., "Gas Separation Modes in a Hollow Fiber Contained Liquid Membrane Permeator", Ind. Eng. Chem. Res., Vol. 31, p. 593 (1992) presented a very detailed study on the separation of mixtures of CO.sub.2 --N.sub.2 and CH.sub.4 --CO.sub.2 using water and 20% aqueous diethanolamine (DEA) solution as liquid membranes. This study also included experimental data and theoretically predicted results for different modes of permeator operation.
The principle of transport of species through the HFCLM is the same as in the ILM. For a non-reactive liquid acting as the liquid membrane, the permeant gas species dissolves at the feed-liquid membrane interface on the outside radius of the feed fiber, diffuses through the liquid membrane and desorbs at the sweep-liquid membrane interface at the outside radius of the sweep fiber. The microporous hollow fibers do not act as membrane barriers as such. Their pore size and wetting characteristics control the maximum allowable pressure difference between the gas stream and the liquid membrane beyond which one of the phases will get dispersed in the other.
By way of example, and in accordance with the present invention, the separation of CO.sub.2 --N.sub.2 mixtures using a HFCLM permeator will be described. In the first example, both sets of highly intermingled microporous hydrophobic hollow fibers have a thin nonporous silicone coating. In the second example, one set of microporous hydrophobic hollow fibers has a thin non-porous silicone coating, while the other set does not. The former is illustrated in the schematic illustration designated FIG. 4 hereinbelow, with the latter being designated FIG. 5, and with FIG. 5 containing one set of silicone coated fibers and one set of uncoated microporous fibers.
The ultra-thin nonporous silicone layer (.sup.- 1 .mu.m) is located on the outer surface of the hollow fibers and it is used to prevent the membrane liquid from entering the pores of the fibers. The ultra-thin silicone coating also acts as a separation barrier since permeabilities of various species through silicone are different; the resistance to the transport of species which the coating provides is however likely to be negligible due to its thinness. In addition, the coating tends to increase the strength of the microporous fibers making them capable of handling pressures up to 200 psig and above. Thus, the feed stream and the liquid membrane can be maintained at high pressures and the strip side at a lower (e.g. atmospheric) pressure, without fiber collapse and certainly without any membrane liquid breakthrough into the permeate fiber lumen. In the technique without a non-porous coating on the fibers having a pore size around 0.03 .mu.m, the maximum pressure differential that could be maintained was around 100 psia. Suitable hydrophobic microporous hollow fibers found useful in application to the present invention are disclosed in Nomura U.S. Pat. No. 4,824,444, assigned to the same assignee as the present invention, and the disclosure of this patent is hereby incorporated by reference.
The technique of the present invention provides a method for the continuous and steady separation of components. In particular, the technique of the present invention does not require batch processing techniques, and hence significant advantages are created and readily available.
One feed gas mixture used in our efforts consists of CO.sub.2 and N.sub.2 with a CO.sub.2 content of either 10%, 20%, or 40%. The feed gas pressure ranges between 60 to 216 psia; pure water and 20% by weight aqueous solution of diethanolamine (DEA) are used as liquid membranes, with water alone being non-reactive with the feed gas. As indicated hereinafter, solutions of diethanolamine (DEA) are reactive with certain feed gases, with the advantages available from the use of such material being discussed hereinafter. Also, these efforts provided information on the selectivity of the liquid membranes under study. In undertaking processes in accordance with the present invention, care is given to the selection of an appropriate liquid membrane. For example, certain processes and/or techniques cannot be used with hydrocarbon membrane liquids which tend to wet the hydrophobic microporous fibers. In other words, the hydrophobic property of the coated fibers is an important consideration in undertaking certain processes in accordance with the present invention. In other instances, these configurations may employ membrane liquids such as N-decane, kerosene, as well as other such materials for recovering various species from natural gas. Those gaseous components such as ethane or other higher species are of importance.
Typically, the liquid membrane pressure is greater than the pressure within the gaseous phase of the feed. In other words, the liquid membrane pressure is typically maintained at a level greater than that of the gaseous phase within the microporous hollow fibers. Additionally, the liquid membrane pressure is typically maintained at a level greater than that within the fluid phase in the sweep hollow microporous fibers. Such an arrangement enhances the stability of the system and contributes to the ability of the system to maintain continuous and steady separation operation.
Therefore, it is a primary object of the present invention to provide an improved separation method and apparatus for achieving separation of mixtures of gases utilizing hollow fiber contained liquid membrane techniques (HFCLM), and wherein the separation is achieved through the utilization of two individual sets of coated microporous hollow fiber substrates, one set containing a feed stream, and the other containing a strip stream, and with the hollow fiber substrates being spaced apart by an immiscible liquid membrane retained outside of the fibers.
It is yet a further object of the present invention to provide an improved gaseous separation operation for separating components of mixtures of gases utilizing improved HFCLM techniques.
Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification, appended claims, and accompanying drawings.